Astronomers have used the world's two most powerful radar
telescopes to make the most detailed images ever obtained for an
asteroid in a near-Earth trajectory.

With an average diameter of about 3.5 kilometers (2 miles),
1999 JM8 is the largest near-Earth asteroid
ever studied in detail. Although this object can pass
fairly close to Earth in celestial terms, astronomers concur that
an actual encounter with Earth is not of concern in the next few
centuries.

The new images, obtained with NASA's Goldstone Solar System
Radar in California and the Arecibo Observatory in Puerto Rico,
reveal that 1999 JM8 is a several-kilometer-wide object with a
peculiar shape and an unusually slow and possibly complex spin
state, said Dr. Lance Benner of NASA's Jet Propulsion Laboratory,
Pasadena, CA, who led the team of astronomers. The images are
available online at http://photojournal.jpl.nasa.gov or
http://echo.jpl.nasa.gov/~lance/1999JM8.html.

"It will take much more data analysis to determine the
object's shape and exact rotation state," Benner said. "But just
from looking at the images we can see that this nearby world is
extremely peculiar. At this point we do not understand what some
of the features in the images are, much less how they
originated."
The asteroid was discovered on May 13, 1999, at a U.S. Air
Force telescope in New Mexico that is part of the Lincoln Near
Earth Asteroid Research Project, managed by the Lincoln
Laboratories of the Massachusetts Institute of Technology. The
discovery provided adequate notice for radar observations to be
scheduled at Goldstone from July 18 to August 8 and at Arecibo
from August 1-9 during the asteroid's close approach to 8.5
million kilometers (5.3 million miles), the equivalent of 22
Earth-Moon distances.

"Although Arecibo is the more sensitive telescope, Goldstone
is more fully steerable, and we took advantage of the
complementary capabilities of the two antennas," noted Benner.
"The discovery of this object weeks before its closest approach
was a stroke of luck," he said. "The asteroid won't come this
close again for more than a thousand years."

Asteroid 1999 JM8 bears a striking resemblance to Toutatis,
a similar-sized, slowly rotating object also studied in detail
with radar, said Dr. Scott Hudson of Washington State University,
who is an expert in using radar images to determine the shapes of
asteroids.

"The fact that both these several-kilometer-wide asteroids
are in extremely slow spin states suggests that slow rotators are
fairly common among near-Earth asteroids," he said. "However,
although collisions are thought to be the primary process that
determines asteroid spin states, we don't know how the slow,
complex states come about."

The radar imaging technique uses transmissions of
sophisticated coded waveforms and computer determinations of how
echoes are distributed in range and frequency, instead of their
angular distribution, as in normal optical pictures. "Our finest
resolution is 15 meters (49 feet) per pixel, which is finer than
that obtained for any other asteroid, even for spacecraft" said
Dr. Jean-Luc Margot, one of the team members from Arecibo
Observatory. "To get that kind of resolution with an optical
telescope, you'd need a mirror several hundred meters across.
Radar certainly is the least expensive way of imaging Earth-
approaching objects."

The images show impact craters with diameters as small as
100 meters (330 feet) -- about the length of a football field --
and a few as large as 1 kilometer (0.6 miles). "The density of
craters suggest that the surface is geologically old, and is not
simply a chip off of a parent asteroid," said Dr. Michael Nolan,
a staff scientist at the Arecibo Observatory. "We also see a
concavity that is about half as wide as the asteroid itself, but
we're not sure yet whether or not it's an impact crater."

This is hardly the first time that radar has revealed a
near-Earth asteroid with peculiar characteristics, said Dr.
Steven Ostro of JPL, who has led dozens of asteroid radar
experiments. Radar studies have revealed a stunning array of
exotically shaped worlds with compositions ranging from solid
metal to low-density carbonaceous rock and rotation periods
ranging from 11 minutes to more than a week. "These are very,
very strange places," he said. "I really envy the coming
generations of space explorers who will visit them."

The radar observations were supported by NASA's Office of
Space Science, Washington, DC. The Goldstone Solar System Radar
is part of NASA's Deep Space Network. The Arecibo Observatory in
Puerto Rico is part of the National Astronomy and Ionosphere
Center, which is operated by the Cornell University under a
cooperative agreement with the National Science Foundation and
with support from NASA. JPL is a division of the California
Institute of Technology, Pasadena, CA.